Materials Science Forum Vols. 561-565

Abstract: We have found that solution-treated or sensitized SUS304L stainless steel transforms isothermally from the γ (fcc)-phase to the α' (bcc) martensite via the ε' (hcp) martensite, that is, the amount of the α'-phase increases with increasing holding time. By applying magnetic field, the α' martensite can be induced in the ε'-plate when the steel has a sufficiently large ε' plate formed beforehand by isothermal holding. However, the α' martensite cannot be induced by magnetic field in the ε'-plate formed beforehand by a deformation-induced transformation.

Abstract: We have studied diffuse scatterings appearing in Ti-(50-x)Ni-xFe (x = 6, 7, 8, 10) alloy, which shows negative temperature coefficient in electrical resistivity below a temperature Tmin. Electron diffraction pattern of each alloy exhibits diffuse scatterings below Tmin on cooling. The intensity maximum of the diffuse scatterings is located at an incommensurate position of g+<ζζ0>, where g represents reciprocal lattice points of the B2-phase. The value ζ is slightly smaller than 1/3 and decreases with increasing Fe content. The diffuse scatterings of the present alloys are well explained by the nesting effect of the Fermi surface in the B2-type structure, i.e., ζ of each alloy agrees with the length of nesting vector in the [110] direction, which is evaluated by a first-principle calculation.

Abstract: A new technique has been developed, which enables one to randomize the basal plane texture formed in wrought magnesium alloys, AZ31B. Rolls with about 60 mm in diameter having wavy surface of 8 mm in wave length and 3 mm in peak to valley depth were used. The peak lines were perpendicular to the roll axis. The alloy sheet specimen of 50 x 50 mm with 2.5 mm in thickness was heated at temperatures ranging from 623 to 723 K, and then immediately wavy-roll formed. The specimen was re-heated at the same temperature, and then wavy-roll formed with rotating at 90 degree. Such a process was repeated at eight times, then finally flat-rolled with a slight reduction. Electron backscattered diffraction analyses showed that the basal plane texture was predominantly formed in the start material, while after the cross wavy roll-forming, accumulation of basal plane tended to be diffusely distributed.

Abstract: High strengths and impact values are required for carburizing steels used for automotive drive train parts. It is well known that the finer-grain microstructures are favorable for these properties. In this study, with a view to effective refining of grain size with controlling the alloying elements, influence of Mn and Cr content on prior austenite grain size after carburizing and repeated quenching was investigated on Ti-modified carburizing steels with ferrite and pearlite microstructure. It was found that the increase in Mn and Cr contents in steels were effective for the grain refining. These effects were attributed to fine ferrite and pearlite microstructure caused by the lowered Ar3 point due to higher Mn content, and retarding the grain growth of austenite by the fine dispersion of carbides in the carburized layer of specimen due to higher Cr content.

Abstract: We synthesized polymorphic diamond directly from highly oriented pyrolytic graphite (HOPG) using femtosecond laser driven shock wave without catalyst. A femtosecond laser pulse (wavelength: 800 nm, pulse width: 120 fs, intensity: 2×1015 W/cm2) was irradiated onto the HOPG surface in air. Crystalline structures of HOPG after the laser irradiation were analyzed using the synchrotron X-ray at the BL13XU in the SPring-8. We found that the hexagonal diamond exists in the HOPG which was irradiated by the femtosecond laser normal to the basal plane.

Abstract: Hydrogen diffusivity (D) and permeability (q) in Pd50-xTi50(Cr, Fe)x alloys were measured at temperature range between 570 and 770 K. During a heating measurement of Pd47Ti50Cr3 alloy, both D and q abruptly increased by factors of 5 and 2, respectively, at 712 K, with a phase transformation from B19 to B2 structures. However, q was far smaller than those in pure bcc metals (eg. V, Nb and Ta), where the hydrogen atoms occupy the tetrahedral interstitial sites. A first-principles calculation revealed that hydrogen atoms occupy the octahedral interstitial sites in Pd50Ti50 with B2 structure. It is concluded that the small hydrogen diffusivity in B2 Pd50-xTi50(Cr, Fe)x alloys is due to the long distance hydrogen jump and the resultant high activation energy for the diffusion.

Abstract: Phase-separation of D03 precipitates in A2 matrix of Fe-Si-V alloys was investigated with TEM. When Fe-14.5at%Si-12.9at%V alloy is aged at 873 K, the phase-separation of cuboidal D03 precipitates occurs and A2 particles newly appear in each D03 cuboid. The A2 particles grow to become plates, then the A2 plates elongate along {100} to reach the A2 matrix, and finally the split of D03 cuboid is realized to form smaller cuboids. When Fe-15.5at%Si-5.0at%V alloy is aged at 873 K, the phase-separation of rod-shaped D03 precipitates occurs and A2 particles newly appear in each D03 rod. The A2 particles elongate along the long axis of D03 rod to reach the A2 matrix, and the split of D03 rod is realized to form thinner rods. The split in each alloy brings the refinement of two-phase microstructure, which is a result of not only the elastic energies but also the chemical free energy.

Abstract: The phase-separation behaviour of γ’ precipitates in Ni-7.1Al-6.7Si alloy was investigated by means of transmission electron microscopy (TEM). When the alloy is aged at 1173K, coherent spherical γ’ particles having ordered L12 structure appear in γ matrix having disordered A1 structure. When the two-phase microstructure of γ + γ’ is aged at 973K, spherical γ particles precipitate in the individual γ’ precipitates. In the course of ageing at 973K, the new γ particles grow keeping the spherical shape, their number gradually decreases and finally γ particles aging at 1173K gradually change their shape from sphere to cuboid, but do not practically change their size, i.e. such phase-separation behaviour brings the decelerated growth of γ’ precipitates.

Abstract: Phase separations of A1 (γ) supersaturated solid solution into γ, cubic-L12 (γ') and tetragonal-D022 (γ") phases were investigated in Ni-V-Ge alloys by means of transmission electron microscopy (TEM). When Ni-15.8at%V-9.0at%Ge alloy is aged at 1073 K, at the early stage of ageing γ" phases are observed in the γ matrix as triangle- or diamond-shaped precipitates. With further ageing, colonies of lamellar structure consisting of two variants of γ" phase are dispersedly formed in the γ matrix and then γ" plates grow along the <110> direction. When Ni-14.5at%V-8.8.at%Ge alloy is isothermally annealed at 1023 K, first spherical γ' particles precipitate homogeneously in the γ matrix, followed by the formation of lamellar structure of γ" phases. In the course of further ageing, the lamellar structure develops, and only γ' particles around the lamellar structure grow and others gradually fade out.

Abstract: It has been found that during the electrodeposition of metallic material a supercooled metallic liquid is being formed that is solidified at the deposition temperature in the form of a crystalline or amorphous phase. These phase transformations are proved by the existence in the electrodeposited metals of metastable structures that correspond to: the amorphous structure of the solidified metallic liquid, the highly defective crystalline structure of the metal quenched from the liquid state, and the intermediate modifications that had appeared during the fast crystallization of the liquid phase of the polymorphous metal. On the basis of this phenomenon there have been developed the technologies for producing of protective texturally-composite tin electrocoat, nanostructurally-texturally-composite chromium electrocoat, and amorphous composite chromium electrocoat on canned-food steel sheet.